1. Field of the Invention
This invention relates to a method for the production of a semiconductor device having electrode lines formed on a semiconductor substrate and more particularly to a method for the production of a semiconductor device provided with highly reliable interconnections and adapted for a Si semiconductor device or a compound semiconductor device.
2. Description of the Related Art
In recent years, the semiconductor devices such as, for example, integrated circuit devices (integrated circuit elements) represented by logic devices have been showing a conspicuous growth in the degree of integration. In consequence of this growing degree of integration, the lines to be used for electrically connecting active elements in such semiconductor devices are naturally expected to have line width decreased to the fullest possible extent. Since these fine lines are required to have high current density and high operating temperature as well, the practice of endowing the semiconductor devices with exalted reliability by forming these lines with a material of highly endurance against electromigration is in vogue.
While the semiconductor devices of this class are required to attain growth also in operating speed, the RC delay poses a serious problem on the way to the growth of operating speed. For the solution of this problem of the RC delay, it is essential that the passivation films be allowed a decrease in dielectric constant and the line materials be given a decrease in electrical resistance. As line materials which meet these requirements, Al or Al alloys, and Cu and Ag which have lower electric resistance than Al and higher activation energy for diffusion than Al have been known to the art.
As means of fabrication for producing fine electrode lines, generally the reactive ion etching (RIE) method and the ion milling method have been known. The Al lines, for example, is at a disadvantage in entraining the problem of suffering the lithography used in the process of fabrication to induce the phenomenon of exudation due to the reflection of light and the problem of disrupting the uniformity of fabrication owing to the precipitation and the presence of grain boundaries in the RIE process. These problems give rise to such inconveniences as impairing the shape of lines and degrading the reliability of interconnections.
Then, in the case of the Cu interconnections, the fabrication thereof as by the RIE method, for example, cannot be easily implemented because the chloride or fluoride of Cu has low vapor pressure. Specifically, an effort to increase the temperature of the semiconducting substrate as a subject matter of fabrication and increase the vapor pressure of the chloride or fluoride results in promoting the reaction of forming a chloride or fluoride to a point where this reaction affects the interior of the lines. Since no existing resist material is capable of withstanding high temperature, the electrode line still defies this fabrication for a decrease line width.
The method of physical fabrication by ion milling entrains the problem of encountering difficulty in the separation and removal of the masking material after fabrication due to ion damage and the problem of readily inducing a short circuit in the electrode line due to re-adhering of atoms removed by iron milling.
In recent years, for the fabrication of wiring in the process for production of the semiconductor devices mentioned above, the interconnect method resorting to the damascene process has been attracting attention and has been forming a mainstream in the fabrication under discussion. Specifically, the chemical mechanical polishing (CMP) technique has advanced to a point where electrode lines can be formed as required in an embedded pattern. Thus, the practice of forming the electrode lines with Al and Cu as the material is now prevailing. According to this method, an insulating film (interlayer film) is formed on a semiconducting substrate provided with an active region such as, for example, the active-region-forming surface of a Si substrate prior to the deposition of metal film and then trenches are preparatorily formed in the region of the insulating film expected to form the electrode lines.
Then, on the surface which has been fabricated to contain the trenches therein, a metal as the material for electrode line is deposited by the ordinary technique of sputtering, collimation sputtering (anisotropic sputtering), or CVD. Thereafter, by a heat treatment, the metal film deposited as described above is caused to flow and fill the trenches and metal film on the space is removed by CMP to complete the formation of electrode lines as required.
In this case, the connections to the active parts or to the electrodes in the lower layers are attained with metallic pieces which are either passed through contact holes formed in the insulating film or embedded in the insulating film during the formation of interconnections. Further, prior to the formation of a metal film for electrode lines, a barrier metal layer is generally formed.
Incidentally, the heat treatment which is intended for flowing the matal film and filling the trenches is carried out (1) subsequently to the formation of the metallic film with the relevant site kept under a high degree of vacuum, (2) under a degree of vacuum below the equilibrium dissociation pressure of an oxide where the site of formation of a Cu or Al film has been exposed once to the atmospheric exposure after the film deposition or in a stream of hydrogen gas after the chamber for the heat-treatment has been evacuated to a high degree of vacuum, or (3) in a forming gas (mixed gas of N.sub.2 and H.sub.2 generally having a H.sub.2 concentration in the range of from 10 to 20%) of high purity where the heat treatment is to be carried out under an atmospheric pressure.
In any case, the heat treatment is carried out in an atmosphere deprived of an oxidizing gas to the fullest possible extent or in an atmosphere of reducing gas.
The heat treatment for the flow is confronted by two problems.
Firstly, as shown schematically in FIG. 25A, generally a metal film is deposited in a thickness amount 1.5 to 2.0 times the depth of trenches 1 for the purpose of increasing the initial amount of accumulation in the trenches 1, for example. During the process of a heat treatment for flow, therefore, the surfaces of the opposed portions of the deposited film (metal film) 3b on the wall defiling a space 2a of the trench 1 contact each other and produce a bridge 3a and, as schematically shown in FIG. 25B, give rise to a void 4 within the trench 1, with the result that the void 4 will persist and obstruct the flow. In the diagram, 2 and 5 each stand for an insulating film made of such a substance as SiO.sub.2 or SiN, for example.
To be more specific about this point, when the metal intended for the interconnect is deposited by a physical vapor deposition, such as sputtering or vapor deposition, since the directions in which the hurled particles impinge on a substrate constitute a cosine distribution, the accumulation of deposited particles on the space 2a between the adjacent trenches 1 grows in the directions of the trenches 1 with the obliquely impinging particles and induce the occurrence of a overhung portion 3b which will obstruct the accumulation of particles inside the trench 1. When the heat treatment is carried out in the presence of the overhung portion 3b which has grown as described above, the adjacent overhung portions 3b are suffered to contact each other in consequence of thermal expansion and the portions of this contact continuously grow (necking) to induce formation of a bridge 3a between the opposed walls of the space 2a. In consequence of the advance of the linkage between the opposed walls of the spaces 2a, the initial empty space remains beneath the region. Since this void 4 cannot be filled by an ordinary heat treatment, the interconnection which is subsequently formed by the CMP is destined to suffer voids to remain therein.
Secondly, even when the bridge 3a mentioned above is not suffered to occur as schematically shown in FIG. 26A, the problem arises that the accumulation in the trench 1 while undergoing the heat treatment for flow is lifted up onto the space 2a between the adjacent trenches 1 and suffered to leave a void 4 behind within the trench 1 as schematically shown in FIG. 26B, with the result that the produced interconnection will suffer from degraded reliability or even sustain disconnection. To be more specific, in this case, the heat treatment causes the metal once accumulated in the trench 1 to move gradually owing to the surface diffusion which originates in the difference in the radius of curvature of surface as shown schematically in FIG. 26A. Since the accumulated film 3 is still in an energetically metastable state at this stage, however, the movement of the accumulated film 3 is further advanced with the decrease in the surface energy and the interfacial energy as the driving force. The direction in which the accumulated film 3 moves in this case is determined by the relation between the amount of accumulation on the space 2a and the amount of accumulation in the trench 1. On the assumption that the particles involved in the accumulation are simple spheres, the reaction proceeds in reverse proportion to the fourth to the third power of the particle diameter. In other words, the movement of the metal film proceeds in the direction from the side of a small amount of accumulation to the side of a large amount of accumulation as schematically shown in FIG. 26B. Further, when the metal is deposited by an ordinary sputtering technique, the possibility arises in an extreme case even before the heat treatment for flow that accumulated films will be bridged between adjacent spaces 2a and void 4 will be formed in the trench 1.
FIGS. 27A, 27B, and 27C schematically show the state in which Cu as the metal for interconnection is embedded in the trench 1 when the Cu is deposited by the conventional heat sputtering technique. Generally, the sputtering of the metal is carried out in the ambience of an inert gas such as Ar gas for the purpose of preventing the magnitude of the resistance of the produced interconnections from being increased by the oxidation of the metal. In the sputtering which is performed on a Si substrate containing the trench 1, the speed of accumulation of the metal is lower inside the trench than on the flat part of the surface. This is because the range of angle permitting incidence of sputtered particles (angle of anticipation) is small in the bottom part of the trench 1 as compared with the flat part of the surface because of the difference in height.
When the formation of the metal film is carried out in this case by the sputtering technique on the Si substrate which is kept in a heated state, the metal during the initial stage of the film formation undergoes aggregation in the form of islands so as to decrease the surface energy as shown in FIG. 27A. The aggregation mentioned above is liable to occur conspicuously on the inner wall of the trench 1 particularly because of a low deposition rate. When the aggregation occurs on the lateral wall part in the trench 1, the islands of metal formed in the opening part of the trench 1 are exclusively allowed to grow preferentially as shown in FIG. 27B because these islands of metal in the opening part decrease the angle of anticipation and obstruct the advance of sputtered particles into the trench 1. As a result, the islands of metal which have preferentially grown from the opposed lateral walls of the opening part come into tight contact with each other and the voids 4 are suffered to persist inside the trench 1 and consequently the trench 1 is no longer enabled to be filled with the deposited film 3 as shown in FIG. 27C.
When the film is formed by the ordinary physical deposition as described above, the two problems mentioned above cannot be solved because the accumulated film on the space 2a has a large thickness as compared with that on the inside of the trench 1 owing to the presence of the overhung parts 3b of the accumulated film on the space 2a. Though the anisotropic film deposition is one method available for increasing the amount of accumulation of film on the inside of the trench 1, it is at a disadvantage in not only attaining the formation of a film only with inferior efficiency but also encountering obstruction in effecting the movement of the accumulated metal from above the space 2a into the trench 1 because the film on the lateral wall of the trench is so thin as to sustain agglomeration and consequent disconnection when subjected to a heat treatment.
Then, in the formation of a buried line by the use of the CMP technique mentioned above, the trenches ought to be accurately formed in conformity with the pattern of the lines. In the photoetching process, therefore, it is necessary that the exposure of the resist to light be prevented from being disturbed by the random reflection of light from the material of the lower layer. In order to preclude the random reflection mentioned above, a TiN layer having low reflectance is formed as an antireflection film prior to the formation of the metal film for electrode line. Further, this TiN layer is concurrently used as a diffusion barrier for such an interconnection metal as Cu which is liable to diffuse in an insulating material. Incidentally, since the compound TiN is a conductor, the unnecessary part of TiN must be removed after the wiring of Cu, for example, has been formed. This removal cannot be readily attained by etching with an acidic solution because the Cu as the interconnection metal has poor resistance to acids. Thus, the removal of the unnecessary part of TiN ought to be effected concurrently by the CMP technique mentioned above.
In the formation of a wiring by the damascene process mentioned above, the removal of the unnecessary part of the metallic film to be effected by polishing at the CMP step ought to proceed so that the material of the lower layer will not be excessively polished. Since the TiN film is so hard, the lower layer ought to be formed of an insulating material having a lower polishing speed than the TiN film in order that the removal may be attained without entraining any excessive polishing of the material of the lower layer. The selection of this material poses numerous difficulties and entails an increase in the number of steps of the process.
To cope with these problems, the feasibility of using a C (carbon) film which has a lower reflectance and a lower polishing speed than the TiN film is now being studied. This is because the C film, when used as an antireflection film, not only represses the degradation of the accuracy of a resist pattern due to the random reflection of light but also fulfills the function of a film for stopping the work of polishing from advancing into the TiN film. Though the C film thus used indeed realizes the removal of the TiN film while avoiding excessive polishing of the material of the lower layer, it must be eventually removed. This removal of the C film can be attained in an oxygen plasma, for example. On exposure to the oxygen plasma, however, the Cu line tends to be oxidized in consequence of the elevation of the temperature of the substrate and then caused to sustain deformation and gain in the magnitude of electrical resistance.
The method for forming an electrode line by the use of the CMP technique mentioned above has been attracting the attention of many scientists engaging in the manufacture of semiconductor devices. It nevertheless has some problems standing on the way to the reduction to practice. In the formation of an electrode line using Cu as the raw material, for example, there arises the problem that the Cu, in the process of heat treatment for flow, will possibly pass through an underlying insulating film, reach a Si substrate, for example, and cause deterioration of semiconductor characteristics. To avoid this problem, the method of utilizing a barrier metal or an interlayer film for preventing the Cu from being diffused in the Si substrate has been adopted. Since no fully satisfactory barrier in this respect exists at present, this method is fated to impose a restriction on the temperature of flowing and fail to acquire a fully sufficient flowing temperature. Particularly when a metallic film which has been formed by an ordinary sputtering technique requires flowing, the aforementioned deterioration of the semiconductor characteristics owing to the diffusion of the metal for an electrode line constitutes a serious problem because the metal film must be heat-treated in a high degree of vacuum at 750.degree. C. for not less than 10 minutes.
As one measure of relief, the feasibility of the collimation sputtering which is capable of exalting the efficiency of the embedment of sputtered particles in the trenches during the stage of film formation by utilizing the straightly advancing part of sputtered particles and eliminating the part of sputtered particles advancing at a large angle of impingement relative to the semiconducting substrate has been tried. This method of collimation sputtering, however, is not fully satisfactory in terms of productivity or mass-producibility because it consists in utilizing the straightly advancing part of sputtered particles and consequently suffers from decisively low efficiency of film formation as compared with the film formation by the ordinary sputtering technique.
As another method for the formation of an electrode line, the means of embedding the metal for the electrode line in the trench by the selective CVD (chemical vapor deposition) is available. This method, however, is at a disadvantage in being expensive in addition to encountering technical problems yet to be solved.